A liquid crystal panel, and a liquid crystal module obtained by adding a driving circuit to the liquid crystal panel, are known in the prior art, in which the liquid crystal panel includes a liquid crystal material interposed between a pair of glass substrates opposing each other, and is capable of displaying various types of visual information such as patterns, characters and symbols by utilizing the nature of the liquid crystal material of changing the light transmittance thereof according to the orientation thereof in response to a voltage applied between the pair of glass substrates.
FIG. 9 is a plan view illustrating a conventional liquid crystal module 100. As illustrated in the figure, the liquid crystal module 100 can be divided into a liquid crystal panel 101 and a driving circuit for driving liquid crystal elements 102 in a liquid crystal display section 101a of the liquid crystal panel 101. A pair of glass substrates interposing a liquid crystal material therebetween are provided in the liquid crystal display section 111a of the liquid crystal panel 101. The liquid crystal elements 102 and TFTs 103 are arranged in a matrix pattern between one of the glass substrates (the upper glass substrate), which is shown in FIG. 9, and the counter glass substrate (the lower glass substrate), which is not shown in FIG. 9. Each liquid crystal element 102 includes a liquid crystal material interposed between a transparent electrode formed on the lower surface of the upper glass substrate and a counter transparent electrode formed on the upper surface of the counter glass substrate, for example. Moreover, each TFT 103 is a transistor connected to the transparent electrode on the lower surface of the upper glass substrate for controlling the voltage of the transparent electrode.
Moreover, the driving circuit includes: a plurality of (eight in this example) source drivers 104 for controlling the respective source voltages of the TFTs 103; gate drivers 105 for controlling the respective gate voltages of the TFTs 103; a voltage production/control circuit 120 for producing voltage signals and control signals to be supplied to the source drivers 104 and the gate drivers 105; a first wiring substrate 110 provided between the voltage production/control circuit 120 and the source drivers 104; and a second wiring substrate 112 provided between the voltage production/control circuit 120 and the gate drivers 105. The first wiring substrate 110 and the source drivers 104 are connected to each other via flexible wires 111, and the second wiring substrate 112 and the gate drivers 105 are connected to each other via flexible wires 113. The source drivers 104 and the gate drivers 105 of the driving circuit are arranged in the liquid crystal panel 101 excluding the liquid crystal display section 111a, thus forming a so-called COG (Chip On Glass) type structure. The source drivers 104 are individually formed respectively on eight LSI chips, for example.
In the liquid crystal panel 101, a large number of data lines 106 extend from the source drivers 104 of the driving circuit along columns shown in FIG. 9 into the liquid crystal display section 111a, and the data lines 106 are connected to the respective sources of the TFTs 103. Moreover, a large number of gate lines 107 extend from the gate drivers 105 along rows shown in FIG. 9 into the liquid crystal display section 111a, and the gate lines 107 are connected to the respective gates of the TFTs 103. The modes in which the voltage to be applied across the liquid crystal element 102 is controlled include, with the voltage polarity when the transparent electrode is at a higher potential than the counter transparent electrode being defined as “positive”: a mode of a first type in which the voltage of the TFT-side transparent electrode is controlled to voltage values in n steps (64 steps in this example) while switching the voltage of the counter transparent electrode to be positive and negative at regular time intervals; and a mode of a second type in which the voltage of the TFT-side transparent electrode is alternately inverted to voltage values in n steps in the positive and negative directions (64 steps each, or 128 steps in total, in this example) at regular time intervals while maintaining the voltage of the counter transparent electrode at a constant level (e.g., the intermediate potential of VDD/2). Both modes are designed so that an error in brightness due to deterioration of the liquid crystal material can be avoided as long as the voltage applied across the liquid crystal element 102 is always of the same polarity.
FIG. 10 is a block circuit diagram schematically illustrating the structure of a conventional source driver 104A of the first type. As illustrated in the figure, the source driver 104A includes therein: pads 133 to which reference voltage wires 131 are mechanically connected; a reference voltage production resistor section 132 for receiving signals from the reference voltage wires 131 to produce subdivided reference voltages; a large number of voltage level selection circuits 134 connected to the reference voltage production resistor section 132; and output buffers 135 arranged on the subsequent-stage side of the respective voltage level selection circuits 134. Thus, voltage-related signals are produced in the source driver 104A as much as possible, with only the reference voltage being externally produced.
The reference voltage wires 131 are wires connecting the voltage production/control circuit 120 to the source driver 104A, some of the reference voltage wires 131 being the flexible wires 111. Note that other than the reference voltage wires, data signal lines (e.g., 6 bits) are also connected to the source driver 104A, and the first wiring substrate 110 has a structure including a number of substrate layers stacked together for supporting the very large number of wires.
The reference voltage production resistor section 132 controls the orientation of one liquid crystal element 102 in n steps (e.g., 64 steps) so as to give n steps (e.g., 64 steps) of brightness. For example, ten reference voltage wires 131 carrying therethrough signals of ten steps of voltage values different from one another are connected to the reference voltage production resistor section 132 so that the ten steps of voltage values are further subdivided into 64 steps of voltage values by the reference voltage production resistor section 132. Moreover, the first wiring substrate 110 described above is for supporting the reference voltage wires 131, etc.
Each voltage level selection circuit 134 receives a voltage signal from the reference voltage production resistor section 132 via n signal lines, and the voltage level selection circuit 134 allows a voltage signal supplied from one of the n signal lines passes therethrough under the control of a voltage selection control signal Svs so that the voltage signal is output to the data line 106 via the output buffer 135. Thus, the voltage to be applied, via the TFT 103, between the pair of transparent electrodes interposing the liquid crystal element 102 therebetween is controlled to be one of 64 steps by using the voltage selection control signal Svs, thereby controlling the brightness of light passing through the liquid crystal element 102. Moreover, for example, 384 voltage level selection circuits 134 are provided in each source driver 104A in a case of a color display.
Moreover, FIG. 11 is a block circuit diagram schematically illustrating the structure of a conventional source driver 104B of the second type. As illustrated in the figure, the source driver 104B includes therein: a positive-side reference voltage production resistor section 132a for receiving a reference voltage whose potential is higher than that of the intermediate voltage applied to the counter transparent electrode; and a negative-side reference voltage production resistor section 132b for receiving a reference voltage whose potential is lower than that of the intermediate voltage applied to the counter transparent electrode. Each voltage level selection circuit 134 can be divided into a positive-side voltage level selection circuit 134a for receiving the output from the positive-side reference voltage production resistor section 132a, and a negative-side voltage level selection circuit 134b for receiving the output from the negative-side reference voltage production resistor section 132b. The positive-side voltage level selection circuits 134a and the negative-side voltage level selection circuits 134b are arranged alternately. The output from the positive-side voltage level selection circuits 134a and the output from the negative-side voltage level selection circuits 134b are alternately switched to one another so as to be supplied to the output buffers 135, 135 provided on the output side thereof, by a selector 136 receiving the outputs from the positive-side voltage level selection circuit 134a and the negative-side voltage level selection circuit 134b according to a selector control signal Sse. Thus, voltage signals that are alternately switched between a high level and a low level at regular time intervals are output from the two output buffers 135, 135 adjacent to each other. Specifically, voltages of the opposite polarities are always applied across the liquid crystal elements 102 connected to the adjacent data lines 106, with the polarities being inverted at regular time intervals. Thus, the source driver 104B provided in a liquid crystal module of the second type switches the voltages of the adjacent data lines 106 between the high level and the low level so that the voltage applied across each liquid crystal element 102 is switched between a positive value and a negative value at regular time intervals.